His next published scientific paper was, "On the Production of Cold by the Rarefaction of Air: accompanied by Experiments." "One of these experiments most strikingly illustrated the great reduction of temperature which takes place on the sudden rarefaction of condensed air. Half a pint of water was poured into a strong copper vessel of a globular form, and having a capacity of five gallons; a tube of one-fourth of an inch caliber, with a number of holes near the lower end, and a stop-cock attached to the other extremity, was firmly screwed into the neck of the vessel; the lower end of the tube dipped into the water, but a number of holes were above the surface of the liquid, so that a jet of air mingled with water might be thrown from the fountain. "The apparatus was then charged with condensed air, by means of a powerful condensing pump, until the pressure was estimated at nine atmospheres. During the condensation, the vessel became sensibly warm. After suffering the apparatus to cool down to the temperature of the room, the stop-cock was opened: the air rushed out with great violence, carrying with it a quantity of water, which was instantly converted into snow. After a few seconds, the tube became filled with ice, which almost entirely stopped the current of air. The neck of the vessel was then partially unscrewed, so as to allow the condensed air to rush out around the sides of the screw; in this state the temperature of the whole interior atmosphere was so much reduced as to freeze the remaining water in the vessel." Other pamphlets followed this publication, but in 1831 a notable paper in the "American Journal of Science and the Arts" brought Henry's name to the front line of discoverers in electro-magnetism. Sturgeon made the first electro-magnet; Henry made the electro-magnet what it is. Says W. B. Taylor, in an address before the "Philosophical Society of Washington:" "The electro-magnet figured and described by Sturgeon consisted of a small bar or stout iron wire bent into a [Inverted U-Symbol] or horse-shoe form, having a copper wire wound loosely around it in eighteen turns, with the ends of the wire dipping into mercury-cups connected with the respective poles of a battery having one hundred and thirty square inches of active surface."